1. Field of the Invention
The invention relates to magnetic article surveillance systems in general, and in particular, to deactivatable coded markers for such systems.
2. Statement of Art
Article surveillance systems using soft magnetic materials and low frequency detection systems have been known since the Picard patent No. (763,861) was issued in France in 1934. Picard discovered that when a piece of metal is subjected to a sinusoidally varying magnetic field, an induced voltage, characteristic of the metal composition, is produced in a pair of balanced coils in the vicinity of the applied field. Today, such systems utilize the harmonics produced by a marker of soft magnetic strip to detect the marker. Due to the nonlinear characteristics of such markers, groups of even and odd order harmonics can be produced simultaneously or individually. Odd order (1, 3, 5 . . . ) harmonics are produced by a symmetrical switching of the B/H loop. Even order harmonics (2, 4, 6 . . . ) are produced by a non-symmetrical switching condition, typically caused by a D.C. magnetic bias internal or external to the material.
The nonlinear characteristics of the soft magnetic material, while not commonly found, can be duplicated is some ferrous alloys by the presence of a magnetic bias. This results in the generation of even and odd order harmonics that duplicates the response of soft magnetic materials, such as Permalloy and the metallic glass products. However, the use of more sensitive detection equipment can add to the probability of false alarms due to ferrous alloys. More sensitive detection equipment also increases the difficulty of effectively deactivating markers, that is, turning markers, "off".
Another limitation of the soft strip low frequency system is that only a single bit of information is available during marker and system interaction. The marker is either in the detection zone, or not. The only other alternative is that the marker is, whether or not in the detection zone, deactivated. While this is not a disadvantage for systems used in theft control, it is an extreme limitation when used for monitoring the flow of a group of differing objects, or even persons, through the detection zone.
Those systems using coded devices for monitoring people and articles in a selected area are quite capable of a large number of codes. Card access systems are a good example. They generally combine a digital network and/or radio frequency circuit to transmit the code. However these devices are too expensive to use either for theft control of low cost items or for inventory control in factories or stores. It is understood that encoded markers can be affixed to or otherwise carried by any article or person, animal, etc. The term "article" is used herein to encompass such possibilities.
A magnetic article surveillance system disclosed in U.S. Pat. No. 4,622,542 differed from the prior art in that the codes utilized are not duplicated by biased ferrous alloys, even accidentally. Further, the coded marker can be embodied in a single element device and can be programmed (code changed) by altering the geometry of or extent of a conductor surrounding a magnetic core. It is detectable at large distances and is not sensitive to spatial orientation within the system. Article surveillance is based upon the detection of phase shifted harmonics generated by markers in a detection zone. The number of codes does not depend on the marker structure but on the phase resolution of the detection system and programming device. Such a system, based upon phase shifted harmonics, has lent itself to an improved system for reliably deactivating magnetic markers, which system is taught herein.
The concept of this invention can best be appreciated in contrast to the teachings of specific and representative patents. The prior art can be broken down into the following classifications of coded markers, which nevertheless share a common deactivation technology: (1) single element; (2) multiple element (3) biased (magnetized); and, (4) unbiased.
A bistable magnetic device is disclosed in U.S. Pat. No. 3,820,090-Wiegand. The marker is in the form of a wire, preferably with a magnetically "hard" magnetized outer shell (having a relatively high coercivity) and a moderately "soft" magnetic core (having a relatively low coercivity). Coercivity is a measure of the external magnetic field strength needed to neutralize the net magnetization of a magnetized ferromagnetic material. The magnetized shell portion is operable for magnetizing the core portion in a first direction, the magnetization of the core portion is reversible by application of a separate magnetic field and the shell is operable to remagnetize the core portion in the first direction upon removal of the separate magnetic field. The device requires a fixed orientation to the interrogation field. The system can produce additional codes only by using multiple elements. Such devices are generally used for close proximity card access systems.
The device disclosed in U.S. Pat. No. 3,747,086-Peterson uses multiple elements to bias a soft magnetic strip. The marker comprises a plurality of ferromagnetic elements including a first element capable of generating a signal containing harmonics of an exciting oscillatory interrogating field and a second element having a coercive force greater than the first element and capable of retaining a state of magnetization when exposed to the interrogation field, such that when so magnetized, a magnetic bias is imposed on the first element to prevent the generation of the harmonic signal. Four possibilities (codes) exist depending on which element is magnetized. However, these codes are easily reproduced in any biased, ferrous alloy. The system is neither unique nor reliable.
The system disclosed in U.S. Pat. No. 3,765,007-Elder uses markers of "n" number of elements with differing AC coercivities to produce "n" number of codes. When the elements are subjected to a periodically varying magnetic field, the magnetization of the elements reverses sequentially at equal intervals of time. Like Peterson, Elder's system is prone to false alarms from biased, ferrous alloys which inadvertently, and all to frequently, duplicate the code. Moreover, a plurality of magnetic field producing means must be used to cover all orientations of the coded elements (markers).
The system disclosed in U.S. Pat. No. 4,134,538-Lagard, et al. uses markers of "n" multiple elements or bands producing varying amplitudes as a code. Such magnetic bands are selectively divided at variable predetermined locations by cuts of variable predetermined extent, such that when in the detection zone, signals of varying amplitudes are produced. The marker must pass correctly oriented and in close proximity to the coils in the detection zone. It is primarily a device intended for access or inventory control and is expensive to produce.
Although U.S. Pat. No. 4,489,313-Pfister discloses a phase detector, it is not the phase of transmitted and returned signals that is determined, but two different parts of the returned signal.
The foregoing references indicate that markers utilizing a single strip of soft magnetic material can be deactivated by placing one or more elements of a high coercivity material along the length of the single strip. A magnetic bias applied to and retained by the high coercivity material reduces the harmonic generation of the single, soft magnetic strip. This technique is often unreliable, and usually is ineffective when the marker encounters the high field intensity of the transmitter and a closely coupled, highly sensitive receiver. Such a technique is also particularly ineffective for preventing false alarms due to the presence of multiple deactivated tags. The deactivation technique of such prior art systems is such that the harmonic signal generated by the soft magnetic material is normally not completely eliminated. Assuming, for example, that a system is effective to reduce the amplitude of the harmonic signal to ten percent (10%) of its normal level, then a consumer carrying ten deactivated tags on ten legitimately purchased articles will likely set off an alarm due to the cumulative amplitudes of the ten damped harmonic signals.
A deactivation system taught by U.S. Pat. No. 3,820,104-Fearon seeks to deactivate tags more effectively by utilizing a high coercivity element capable of imposing a plurality of pairs of alternate magnetic poles on the soft magnetic material. Even assuming that such a technique is more effective in preventing generation of harmonic signals by markers, such harmonic signals are nevertheless still generated at low amplitude levels. A sufficiently large number of deactivated markers can still be expected to generate a false alarm condition.
The invention disclosed in U.S. Pat. No. 4,622,542, the teachings of which are fully incorporated herein by reference, is based upon the discovery that when a suitable conductor, such as aluminum or copper, partially or totally encloses a core of soft magnetic material, the phase of the harmonics produced will be shifted (delayed in time). The amount of phase shift induced is controlled largely by the amount and resistivity of the conductor surrounding the magnetic material. It is feasible to shift any harmonic or groups of harmonics by any amount, through 360 degrees. However, some loss of harmonic amplitude is encountered as the conductor thickness increases and as the harmonic number increases.
The ability to control harmonic phase permits the generation of signals having a unique signature, apart from both ferrous alloys and soft magnetic materials. This avoids the accidental detections plaguing prior art systems as described above. In addition, a number of codes can be established according to the phase shift induced. The phase shift is not affected by a low level, external magnetic bias, in that odd order products are totally unaffected and even products shift by .+-.180 degrees.
In a very simple embodiment of a deactivatable tag according to this invention, which tag is suitable for use in a phase shift dependant magnetic article surveillance system, a tag or marker comprises a core of soft magnetic material surrounded by a ring of electrically conductive material. A second ferromagnetic element, having a higher coercivity than the core material, is placed over all or a portion of the encircling conductive material. In accordance with the teachings of U.S. Pat. No. 4,622,542, the conductive material is responsible for a predetermined phase shift of the harmonic signal generated by the marker in a surveillance or detection zone. However, it has been discovered that whenever the higher coercivity magnetic material is itself magnetized by an external magnetic field, it has the effect of shutting off that portion of the marker (i.e., the core) so that the harmonic signal is not affected by the conductive material. In effect, the core becomes blind to the presence of the conductive material and the harmonic signal is not phase shifted by the predetermined amount necessary to constitute an alarm condition. According, the problem of false alarms due to the presence of multiple deactivated tags is eliminated altogether, as the reduced phase shifts of deactivated tags are not cumulative. In other words, ten harmonic signals, each of which is shifted by only ten degrees, rather than for example by 100 degrees, will not appear to be cumulatively shifted by 100 degrees.
Development of deactivatable coded markers according to this invention brought to light a potential difficulty with multiply coded markers. The phase of odd order marker generated harmonics is dependant upon the saturation characteristics of the soft magnetic material, and the saturation characteristic changes somewhat as the field intensity of the transmitted signal varies. In other words, when a coded marker enters a detection zone, the intensity of the transmitted electromagnetic field tends to vary. The variation in intensity causes the phase of the odd harmonics to shift to some degree. This does not present a problem for basic kinds of markers coded only for on or off theft detection systems, but eventually causes a problem when high resolution is needed in order to decode relatively small phase shifts. It has been discovered that this problem can be solved by incorporating into an article surveillance system, operating on phase shifted harmonics, means for automatically adjusting the transmitted reference signals responsive to a characteristic of the received harmonic signals to compensate for the variation due to marker presence in the detection zone and thereby prevent the random variation in the phase shift of the harmonic signals. Presently, it is preferred that the automatic adjusting means alter the intensity of the transmitted reference signals responsive to the intensity of the received harmonic signals, and in particular, those received signals corresponding to the third harmonic. Such means, which can be embodied in an automatic gain control circuit, automatically adjust the intensity of the transmitted signal to maintain a constant harmonic intensity level in the receiver, which results in very stable harmonic phase angles. Such stable harmonic phase angles enable very high resolution and very large numbers of codes.